--> Micrite, Microporosity and Total Organic Carbon Content: Depositional and Diagenetic Controls Linking Small- and Large-Scale Observations in the Tuwaiq Mountain and Hanifa Formations

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Micrite, Microporosity and Total Organic Carbon Content: Depositional and Diagenetic Controls Linking Small- and Large-Scale Observations in the Tuwaiq Mountain and Hanifa Formations

Abstract

Micrite texture, porosity development, and total organic carbon (TOC) content are interrelated in carbonate mudrocks, and have been examined in this study, within a sequence stratigraphic framework in the Tuwaiq Mountain and Hanifa formations, Saudi Arabia. Tools used are nuclear magnetic resonance logs, scanning electron microscopy, and confocal microscopy. Nuclear magnetic resonance is suitable for defining pore-size distributions. Confocal microscopy is most suitable for quantifying and examining microporosity, micrite texture, and examining larger-scale heterogeneity (centimeter- to 10s of centimeters-scale). Detection of organic matter is possible, but organic pores are beneath the resolution of the tool. Scanning electron microscopy (SEM) is most suitable for studying micrite texture and organic porosity. Results show that carbonate composition is the dominant control on micrite texture. High-magnesium calcite and aragonite grains recrystallize into low magnesium calcite, which leads to fusing of micrite grains, and porosity reduction. Organic matter, if present, inhibits fusing. The Tuwaiq Mountain Formation wackestones are composed of coalesced micrite whereas the packstones are composed of cemented micritized shelf-derived grains. Both facies contain euhedral pyrite. The Hanifa Formation contains both tight and relatively porous micrite particles. The highest TOC values were recorded in the transgressive system tracts which are mainly composed of dark laminated mudstones. Framboidal pyrite is commonly associated with organic matter. Porosity types found are interparticle, intraparticle in pyrite, organic pores and fractures. SEM images show that coccoliths are the most prominent grain type in the darker-colored intervals of the Hanifa Formation. These darker-colored intervals are more pervasive in the transgressive systems tracts. Coccoliths can bind with fecal pellets to increases settling velocity, thereby helping to preserve organic matter. Lighter-colored intervals, which are dominant in highstand systems tracts, are composed of more euhedral, relatively fused micrite grains. These grains probably formed due to recrystallization of metastable carbonate grains that were deposited as gravity flows derived from the shelf. The sequence stratigraphic framework has provided a framework for micrite texture, porosity, and TOC content prediction.